Great analysis! While being a silent protagonist results in shortcoming in terms of character exploration potential, I honestly wouldn't have it any other way. While it would be nice to see his writing completely developing into four different entities, it also makes him hard to identify and distinguish from each other since people more often than not lump all the characterisation together, and honestly making it so that his character development is mostly up to speculations based on implications is just better overall. There's a reason why Corrin was almost only recognise for the one side that people just hate on, even though there's technically three versions of him, and one of him is just really straightforward at worst.

also, had Byleth been written into a full character I would have had even more reasons to dislike fem Byleth because an awful fanservicey design is NOT what you are supposed to expect from a canonically stoic character